Phasing and Inheritance

The Ancestry Composition report uses DNA you inherited from recent ancestors on both sides of your family and estimates the proportion of your DNA that comes from each of 45 worldwide populations, offering a detailed view of your ancestry.

Ancestry Composition is even more powerful if you have a biological parent who is also in the 23andMe database and sharing with you. By comparing your parents' DNA to your own, Ancestry Composition can determine how your DNA was inherited, and display which portions of your ancestry came from which parent.

Connecting with a parent may also increase the resolution of your assignments. That translates into better Ancestry Composition results, in the sense that you might see more assignment to the fine-resolution ancestries: for example, more Scandinavian and less Broadly Northern European.

Getting Started

After you have connected with a close relative, your updated results will be available within a few (typically three to five) business days. You can confirm this update by checking the Change Log in the Scientific Details section of the Ancestry Composition report. This log will update and display when phasing was completed and which relatives you are phased against; one parent or both parents:

Calculating Inheritance

Summary: If Ancestry Composition only used your autosomal chromosomes to calculate Inheritance, both sexes would share 50% with each parent. The inclusion of the X chromosome creates an uneven split for males.

It is not uncommon for Ancestry Composition Inheritance to report that a son or daughter inherited slightly more or less than 50% from each parent. This is because Ancestry Composition relies on the autosomes (chromosomes 1–22) and the X chromosome(s) to calculate Inheritance. While the autosomes are passed down equally from each parent for both men and women, the X chromosome isn’t passed down equally to both sexes:

The autosomes (chromosomes 1–22) are passed down equally from each parent; one copy from each parent. Therefore, a parent and a son or daughter would share 50% of their autosomal DNA.

The X chromosome is one of the sex chromosomes (X Y); these chromosomes determine if you’re a female (XX) or a male (XY). Women inherited two copies of the X chromosome—one from each parent—while men inherited one X from their mother and one Y from their father.

Ancestry Composition Inheritance assigns every phased autosomal chromosome in the chromosome pair equally to each parent. For men, the phased X chromosome is only assigned to the mother, since men don’t inherit an X chromosome from their father. Therefore, the mother's assignment includes the length of an extra chromosome that the father's does not. As a result, Parental Inheritance for men is calculated as greater than 50% inherited from the mother and less than 50% inherited from the father.

Technical Details

Your 23 chromosomes come in pairs; one chromosome in each pair comes from your mother, and the other from your father. Genotyping chips—like the one 23andMe uses—don't capture the information about which genetic information or alleles came from which parent; DNA data from our genotyping chip is called "unphased."

Since runs of markers are more informative about geography than are individual markers, we first need to determine which alleles were inherited from each parent and are on the same chromosome together, through a process called "phasing." You could think of phasing as sorting which DNA you inherited from each parent.

The Ancestry Composition analysis includes a statistical, population-based phasing component for all customers. However, when you connect with a biological parent you'll get a very high-quality chromosome phasing result. This translates into better Ancestry Composition results, in the sense that you might see more assignment to the fine-resolution ancestries: more Scandinavian, less Northern European.

Your results get that boost in resolution if you connect with either parent.It's slightly better to use two parents than one parent—however, you get the majority of the benefit with the first relative you add.

If you connect with one or both of your biological parents, you will get an extra result. You'll be able to see the Parental Inheritance view, which shows the contribution of your mother to your ancestry on one side and the contribution of your father to your ancestry on the other. We can't provide this view if you don't have a parent connected because while statistical or population-based phasing allows Ancestry Composition to sort out what you inherited from parent 1 and from parent 2, without at least one biological parent to compare against, the feature can’t determine if parent 1 is your mother or father.

Common Questions

Can I use a son/daughter, sibling, aunt/uncle, or grandparent in place of a parent?

No, the Ancestry Composition algorithm can only close-relative phase when a biological parent is connected. You may be able to learn a little bit more about your ancestry, even if you are not connected with either of your parents through 23andMe, by looking at the location of segments in your Chromosome Painting or comparing compositions with your relatives. Learn more.

How long does it take to receive Parental Inheritance?

Typically 3–5 business days. Make sure that you are connected to a biological parent via sharing or are located in the same account, and that everyone has received their initial results. If the sample for your parent is still in process at the laboratory, Parental Inheritance won’t be available until after his or her sample is finished processing.

Why would Parental Inheritance display less than 50% from a parent?

For men, Parental Inheritance will show greater than 50% inherited from the mother and less than 50% inherited from the father. If Ancestry Composition only used your autosomal chromosomes to calculate Inheritance, both sexes would share 50% with each parent. The inclusion of the X chromosome, which men only inherit from their mothers, creates an uneven split for males.

Which close relative should I connect with to get the Parental Inheritance view?

The Parental Inheritance view—which displays which portions of your ancestry came from which parent—is only available when a biological parent is connected. It's slightly better to use two parents than one parent, however, you get the majority of the benefit with the first parent you add.

Which close relative should I connect with to get the better resolution?

Your results get that boost in resolution if you connect with either parent. It's slightly better to use two parents than one parent—however, you get the majority of the benefit with the first relative you add.